In WO2009/102197, which is incorporated herein by reference, firing line hoist systems to be mounted on the hull of a monohull drilling vessel are disclosed. In general these known firing line hoist systems comprise:                a tower, which tower has a top side and a base, a first side and a second side, and wherein the base of the tower is connected to the hull of the drilling vessel at opposite sides of a moonpool, such that the first side of the tower and the second side of the tower bridge the moonpool in a lateral direction thereof, and        a hoisting device, which hoisting device comprises a load attachment device displaceable supported by the tower, one or more cables and one or more associated winches to manipulate the position of the load attachment device relative to the tower in a vertical direction along a firing line, which firing line extends on the outside of and adjacent to the first side of the tower and through the moonpool, and which hoisting device is adapted for raising and lowering a BOP (Blow Out Preventer) to the seabed;        a BOP transport system, configured for moving a BOP between a storage position and a launch position, in which launch position the BOP is located in the firing line adjacent the first side of the tower, and in which launch position the BOP can be manipulated by the hoisting device, wherein the BOP transport system comprises two parallel guide rails and a BOP transport cart which is movably supported by those guide rails; and        a mobile working deck that is provided at the first side of the tower, and which working deck in an active position extends in a substantially horizontal direction and covers a portion of the moonpool, while the firing line extends through the working deck, and in a non-active position provides room for a BOP to be moved by the BOP transport system into a launch position above the moonpool.        
By arranging the mobile working deck in a mobile manner the deck can be provided in a relative low position with respect to the waterline when work has to be carried out on the working deck, e.g. assembly of a riser string or a drill string. The mobile working deck is removed when a BOP is to be positioned in the firing line. After the BOP has been positioned in the firing line, and has been lowered, the mobile working deck is returned into its active position.
In case the mobile working deck is a sliding working deck, after the BOP is brought into the firing line the BOP is lowered onto a cart positioned above the moonpool. The mobile deck is returned into its active position. The BOP may then be attached to a hosting device and raised with its top end to the level of the working deck, so as to suspend the BOP from a suspension device of the working deck. Subsequently a riser section is connected to the top end of the BOP, and the BOP and riser section are lowered by means of the hoisting device into the sea. This is an elaborate process.
When the mobile working deck is provided with lifting means, the mobile working deck has to be lifted along the tower of the firing line hoist system, such that the BOP can be brought underneath the mobile working deck. The tower is preferably provided with a deck holding device at an elevated position along the tower to hold the deck in the raised, non-active position. It is noted that the working deck is configured for supporting substantially heavy loads, and therefore is quite heavy itself. The deck holding device and the tower should therefore be dimensioned to support the heavy working deck at a substantial height above the main deck. Furthermore the mobile deck may be arranged pivotally, so that e.g. it can be brought in a vertical non-active position. For instance the mobile deck may have two pivotable deck parts.